15.1 Chemistry Of Haloalkanes
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What are haloalkanes?
Compounds containing Carbon, Hydrogen, and at least one Halogen
Prefix for Fluorine
Fluoro
Prefix for Chlorine
Chloro
Prefix for Bromine
Bromo
Prefix for Iodine
Iodo
What type of bond is the Carbon-Halogen bond?
Polar Halogen is more electronegative than the Carbon Halogen will have slightly negative charge Carbon will have slightly positive charge
What are species that donate a lone pair of electrons called?
Nucleophiles
What is a nucleophile?
An atom or group of atoms that is attracted to an electron deficient Carbon atom, where it donates a pair of electrons to form a new covalent bond.
What types of substances are nucleophiles?
Anions (negatively charged ions) Molecules with lone pairs
What happens when a haloalkane reacts with a nucleophile?
The nucleophile replaces the halogen in a substitution reaction
What is a substitution reaction?
A reaction where one atom or group of atoms is replaces by another atom or group of atoms
What is hydrolysis?
Chemical reaction involving water or an aqueous solution of a hydroxide that causes the bond breaking of a molecule
What happens in the hydrolysis of a haloalkane?
The halogen atom is replaced by an OH group
Steps for a nucleophilic substitution reaction
The nucleophile, OH-, approaches the Carbon atom which is attached to the halogen from the opposite side to the halogen atom The direction of attack by the OH- ion minimises repulsion from the halogen atom which has a slight negative charge Lone pair of electrons on the OH- ion is attracted and donated to the Carbon atom New bond is formed between the oxygen atom of the hydroxide ion and the Carbon atom The Carbon-halogen bond breaks by heterolytic bond fission New product - alcohol is formed, as well as a halide ion
How does a nucleophilic substitution reaction take place?
Using aqueous Sodium Hydroxide Reaction is slow at room temperature, so mixture is heated under reflux to obtain a good yield of product
What does the rate of hydrolysis depend on?
Strength of the Carbon-halogen bond
Which Carbon-halogen bond is the strongest?
C—F
Which Haloalkane reacts the fastest?
Iodoalkanes, as bond requires less energy to break
Which haloalkane is unreactive?
Fluoroalkanes as a large amount of energy is required to break the C-F bond
How can the rate of hydrolysis of haloalkanes be measured?
Carry out the reaction in the presence of aqueous silver nitrate As the halide ions are produced during the reaction, they form a precipitate of the silver halide
Are haloalkanes soluble in water?
No
When haloalkanes react with water, what is used?
An ethanol solvent - allows water and haloalkane to mix and produce a single solution
Practical steps for haloalkanes
3 haloalkanes used 1-chlorobutane, 1-bromobutane, 1-fluorobutane 1) Add 1cm3 of ethanol and two drops of each haloalkane into 3 test tubes 2) Place the test tubes in a water bath of 60oC 3) Place a tube of silver nitrate in the water bath as well, so all substances reach a constant temperature 4) Add 1cm3 of silver nitrate to each test tube, start stop watch immediately 5) Observe test tubes for 5 minutes and record time take for precipitate to form
Observations from hydrolysis of haloalkanes
1-chlorobutane - white precipitate forms very slowly 1-bromobutane - cream precipitate forms at medium speed 1-iodobutane - yellow precipitate forms rapidly